Agreed, switching from 8 to 9 speed components does get costly. The thing to remember, there is no difference with regards to the rear derailleur. An 8 will work with 9. There is a slight difference in the front derailleur. A 9 is slightly more narrow than an 8. I've used each back and forth and will work, just not as well as the appropriate derailleur. i.e., I've used an 8 F-der in 9-groupo, and also a 9 F-der in an 8-groupo. It's not optimum, but it'll work!
The Rear Shifters will give you a problem, b/c that's what dictates the amount of cable pull for each shift. The front is the same for 8 or 9.
Chains and chainrings will also be your nemisis. You can use an 8 speed chain on 9 speed rings, but not the other way around.
Arrgggghhh, this is getting complicated. If you want to upgrade to 9 speed get a new bike!
__________________
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW, What a Ride!" - unknown
"Your Bike Sucks" - Sky Yaeger